Venomous fish have evolved many ways to inflict pain

Biologist Leo Smith held an unusual job while an undergraduate student in San Diego. Twice a year, he tagged along on a chartered boat with elderly passengers. The group needed him to identify two particular species of rockfish, the chilipepper rockfish and the California shortspine thornyhead. Once he’d found the red-orange creatures, the passengers would stab themselves in the arms with the fishes’ spines.

Doing so, the seniors believed, would relieve their aching arthritic joints. Smith, now at the University of Kansas in Lawrence, didn’t think much of the practice at the time, but now he wonders if those passengers were on to something. Though there’s no evidence that anything in rockfish venom can alleviate pain — most fish stings are, in fact, quite painful themselves — some scientists suspect fish venom is worth a look. Studying the way venom molecules from diverse fishes inflict pain might help researchers understand how nerve cells sense pain and lead to novel ways to dull the sensation.
Smith is one of a handful of scientists who are studying fish venoms, and there’s plenty to investigate. An estimated 7 to 9 percent of fishes, close to 3,000 species, are venomous, Smith’s work suggests. Venomous fishes are found in freshwater and saltwater, including some stingrays, catfishes and stonefishes. Some, such as certain fang blennies, are favorites in home aquariums. Yet stinging fishes haven’t gotten the same attention from scientists as snakes and other venomous creatures.
But thanks to Smith’s recent work, scientists can now see how venomous fishes fit within a tree of all fishkind. The tree shows that venom arose multiple times throughout history. Understanding which fishes are venomous is the crucial first step to working out the nature of the venoms, Smith says. Researchers are exploring how different fish venoms affect their victims and are discovering extraordinary diversity among fishes’ chemical weaponry. The scientists hope the powerful molecules in the venoms might yield insights that could be turned into medicines. One newly described venom appears to act on opioid receptors, perhaps to stupefy its victims. And venom molecules that stall cell division and others that calm inflammation are inspiring new treatment ideas that go beyond pain relief.

Building a tree
While fish-venom studies are rare, fish stings are not. An old estimate says about 40,000 to 50,000 people are stung by fish each year. But the number is probably much higher, Smith says, since many people don’t bother to report their experiences.
The most noticeable effect of a venomous fish sting is immediate pain, ranging from the mild sting of those rockfish from Smith’s scouting days to a feeling much more excruciating.

“The most pain that I’ve ever been in was my first stingray envenomation,” says venom researcher Bryan Fry of the University of Queensland in Brisbane, Australia. He was trying to collect a sample from a roughly 1½-meter-wide smooth stingray when it stabbed him in the thigh. “The pain is immediate and blinding.”

Smith’s first painful run-in was with a fuzzy dwarf lionfish at a pet store where he worked in his late teens. Later, at the library, he found no reports of that species having venom. In fact, medical records of fish stings documented only about 200 fish species as venomous. The experience helped set his career.

As his research progressed, Smith began building fish family trees to get a better handle on which fish spew venom. He presumed that fish related to known venomous ones could also be venomous. So he checked their anatomy for venom-delivery structures, like grooved spines. He reported a partial tree in 2006 and published a more complete version last year in Integrative and Comparative Biology. To assemble the latest tree, Smith and colleagues examined eight locations in the genetic instruction books, or genomes, of 388 species of fish, then used a computer program to work out, based on differences and similarities in those genomes, how the animals are probably related. He also examined museum samples of 90 types of fish for spines or fangs and venom glands. Based on what’s known about fish diversity, Smith’s lowball estimate is that, of about 35,000 fish species, 2,386 to 2,962 are venomous.
Based on his new tree, Smith estimates that there were 18 distinct instances in which nonvenomous fish evolved a venom apparatus — give or take a few, since venom might have been lost from some groups, or evolved multiple times in others, he says. Jeremy Wright, curator of ichthyology at the New York State Museum in Albany, who has studied venom in catfish family trees, says Smith’s methods were sound and the data support the tree. However, Wright’s research suggests venom arose separately two or more times in the catfish lineage, while Smith’s tree says all stinging catfishes share a single common, venomous ancestor.

Whether fish venom arose 18 times, or 15, or 20, that’s a big contrast to other animals that use venom: In snakes, venom appears to have evolved only once. The same is true for the venom in bees and ants. “To have venom evolve multiple times within a group is extraordinary,” says Fry, who’s studied a range of venomous critters.

Fish experts say the distinct origins of fish venoms make sense because, unlike snakes, which always use their teeth, fish deliver venom in diverse ways. Spines with venom glands are most commonly found in fins atop the fish’s back, but not always. In many venomous catfishes, the pectoral fins contain the barbs and venom glands. Weever fish spines sit on the operculum, a bony flap that protects the gills on the fish’s cheeks. In stingrays, the flattened spine protrudes just above the tail. And in fang blennies, the venom glands sit at the base of enlarged lower canines, calling to mind tiny vampires of the sea.

Even within one fish genus, the venom-delivery apparatus can vary. Ichthyologists Jacob Egge, now at Pacific Lutheran University in Tacoma, Wash., and Andrew Simon of the University of Minnesota analyzed pectoral stingers of 26 species of madtom catfish, found in eastern North American freshwater. Some had smooth spines with a venom gland in the shaft, the two reported in 2011. Others had serrated spines, the better to cause injury, with a gland at the shaft and glands spread along the serrations. One species had no venom gland at all.

The effects of venom — from fishes and other creatures — vary widely, but in fishes, the goal is usually the same: to stop an attack. For most fish venoms, pain is key, but some cause numbness, too. All affect the cardiovascular system in some way, by lowering blood pressure, for example, which would probably startle and debilitate a predator, Smith says.

In people who have been stung, skin reddening, swelling, itching or temporary localized paralysis might also occur. In some cases, the venom can kill the tissues near the sting site. In rare cases, a combination of low blood pressure, failure of circulation or weak breathing can lead to death.

Just within the catfishes, venom effects differ between species. Wright injected venom from nine different species of catfish into largemouth bass, which are typical predators. “It was clear that it was an uncomfortable experience for them,” Wright says of his unlucky subjects. Many venoms caused loss of color and bleeding, some induced jerky muscle contractions or loss of balance, and one simply killed the bass outright, he reported in BMC Evolutionary Biology in 2009.
Modus operandi
Why did such diverse venoms and delivery apparatuses evolve so many times in fish? With Smith’s comprehensive map of fish venom evolution, scientists can now address that sort of question, says Meg Daly, who studies sea anemone venom at Ohio State University in Columbus. For instance, since most fish use venom for defense, Daly wonders if the evolutionary origins of venoms coincided with times when new predators arrived on the scene.

Venom seems to have arisen often in slow-moving bottom dwellers, which would certainly be vulnerable to predation. “If you’re a catfish sitting there sucking on some mud, you need to have some spines,” Fry says.

Consider the reef stonefish. It loafs on the floor of the Indian and Pacific oceans, often covered in camouflaging algae, hoping to snatch a passing fish or crustacean. When distressed, the fish raises the 13 spines on its back that are adjacent to venom glands, which hold toxins powerful enough to kill a person.

Though venoms have evolved multiple times across fish species, the toxic blends often converge chemically on a set of similar ways to cause damage. For example, the proteins in many fish venoms act by assembling into large rings that then insert themselves into the membranes of cells. This opens a hole where a cell’s innards leak out. When this happens to pain-sensing nerve cells, the body interprets the signal as excruciating discomfort — a good way to distract a predator from chowing down, Fry says.

Other fish venoms share their modes of action with certain venoms from other animals. For example, the venoms of stonefishes, snakes and some other organisms contain hyaluronidase, an enzyme that dissolves some of the matrix that supports cells. In that way, the enzyme helps the other venom molecules speed through the victim’s tissues.

But still, the multiple evolutions of fish venoms mean that each group of venomous fish probably makes venom components that attack their victims differently. Scientists are just beginning to delve into the specific molecules and actions of different venoms. Fry and collaborators took a stab in a study published February 16 in Toxins, extracting and analyzing venom from six types of fishes — dusky flathead, Luderick, mullet, yellowback seabream and two types of stingrays.

“It was incredibly variable,” says study coauthor Nicholas Casewell, a venom biologist at the Liverpool School of Tropical Medicine in England. Injected into a rat, the mullet and seabream toxins caused heart rate to drop slightly, while the other venoms had no effect. All venoms resulted in an initial drop in blood pressure — as is common in human envenomations by fish — but the stingray, mullet and seabream venoms then caused blood pressure to rise.

In nerves and muscles growing in a dish, the venoms of stingrays and dusky flatheads blocked muscle twitching, which could potentially mean some moderate level of partial paralysis for a predator, Casewell says. Indeed, paralysis and weakness can occur in people stung by fish. The other fishes’ venoms, in contrast, boosted twitching a bit.

Even though the venoms all cause pain, Fry says, these results show that the underlying effects of each venom are a bit different. It’s a classic case of evolutionary convergence, in which different evolutionary pathways lead to the same end result — in this case, the pain that makes the predator skedaddle.
In a separate study published online March 30 in Current Biology, Casewell, Fry and colleagues examined fang blennies. Certain species, found in shallow reefs of the Indian and Pacific oceans, use venomous fangs to defend against predators. The researchers were puzzled that fang blenny venom didn’t seem to cause pain when injected into a mouse’s paw. The venom, it turns out, acts on opioid receptors, where it might work like a sedative. It also lowers blood pressure, probably leaving the victim disoriented or dizzy. The victim is essentially “stoned,” Fry says. A predator won’t be able to swim away properly, he surmises, or it’ll die of something akin to a heroin overdose.

Another group, at the University of Tübingen in Germany, is investigating the venom of the lesser weever fish of the Mediterranean. Graduate student Myriam Fezai was inspired to study the fish by its ability to induce swelling and paralysis in fishermen and tourists in her homeland, Tunisia. The venom also blackens and kills tissues, so she and collaborators wanted to know how it killed cells. The researchers tested the venom on red blood cells in the lab, where it caused the cells to shrink in a form of programmed cell death, Fezai and colleagues reported in Scientific Reports in 2016.

The team tested the weever fish venom on cancer cells, too. The cells stopped growin g and their mitochondria stopped working properly, triggering apoptosis, a classic mechanism by which cells kick the bucket. Even cells that survived tended to stop dividing regularly. Next, the researchers hope to identify the individual components of the venom involved in the cell killing.

Hurts so good
The hope is that something in weever fish venom can be turned into an anticancer drug. Medicines based on venoms from other animals already exist, including the blood pressure drug captopril (Capoten) from a pit viper. There’s even a painkiller, ziconotide (Prialt), developed from the potent venom of a marine cone snail. Sometimes, the same molecules that cause pain can, if applied correctly, also relieve it. Capsaicin, the spicy tongue-burning stuff in peppers, is used in a cream to relieve the pain of shingles and other conditions. The molecule desensitizes the pain sensors in nerve cells.

Venoms provide a rich source of potentially useful molecules, says Mandë Holford, a snail venom expert at Hunter College and the American Museum of Natural History in New York City. Evolution has already honed the venoms to precisely interact with their targets. “Every time I read about a new venomous organism, like the fish in Leo [Smith’s] work, I get excited becaus e our pot is getting bigger,” she says.
Killer medicines
Several venoms, examples below, have been repurposed as medicines for human use, most for their effect on blood.

Scientists around the world are in the early stages of investigating fish venoms that might combat cancer, control blood pressure or clot blood. In Brazil, researchers studying the venom of the lagoon-dwelling toadfish Thalassophryne nattereri found a small protein they named TnP, which has anti-inflammatory abilities. They hope to develop a medicine for multiple sclerosis, a disease in which immune cells cause inflammation and attack the nervous system. In February, the team reported in PLOS ONE that in mice with a form of multiple sclerosis, a synthetic version of TnP dampened inflammation, protected and promoted repair of nerves and improved muscle coordination.

Isolating the specific venom ingredient that causes the desired effects, as the Brazilian researchers did with TnP, is the direction several scientists are going in their studies of fish venom. Some are analyzing which genes are uniquely turned on in a fish’s venom glands and not activated in nearby fin tissue. Modern mass spectrometry also helps, Holford says, because it allows scientists to analyze the components of even the tiny amount of venom they can extract from a snail or fish.

Unlike snakes, which are easily milked for their venom, collection from fish typically involves clipping the spine off wild specimens and scraping a small bit of venom into a test tube. (The involuntary donor, sent on its way, can typically regrow the spine, like a fingernail, Fry says.) Then things get difficult.

“Fish venom is just horrible … it has this snotlike consistency,” Fry says. “It’s easily the most challenging venom that I’ve had the misfortune to work with.” In contrast to venom from other creatures, which often consists of fairly small, stable proteins, fish venom tends to be made of large proteins that fall apart easily once out of the fish. Freeze it, heat it or expose it to certain chemicals, and the proteins fall apart. That’s a major disadvantage in the lab, and for medicines, too, Casewell notes. Therefore, he doubts a fish venom could yield the next blockbuster pharmaceutical.

Fry acknowledges that successes such as captopril or ziconotide, in which venom directly leads to a medicine, are quite rare. However, he believes scientists can learn about pain from fish venoms and apply that knowledge to invent novel painkillers. Similarly, Fezai, who started the weever fish project, doesn’t think the venom ingredients themselves would be the drug, but some molecule that mimics their actions might be.

The upside of the fragility of fish venoms, though, is that treatment for a fish sting is quite straightforward: running hot water over the affected body part. That’s what Smith did when he was stung by a blue tang — think Dory from Finding Nemo — while cleaning his tank at home. About a half an hour under the hot tap stopped the pain by destroying the venom in his finger. But some damage had already been done. About 10 days later, a pea-sized chunk of his finger fell off, dead.

The rockfish, so desired by Smith’s copassengers on the San Diego fishing trips, has a milder sting. Those arthritis sufferers weren’t risking much. But whether they were really relieving joint pain with a fish venom is an open question. They certainly seemed to think so, Smith says, though as of yet no data support this particular fishy treatment.

But, he notes, the venom of scorpion fish — cousin to rockfish — affects the nervous system, immune system and blood pressure, all of which could, in theory, have some “real” effect on the arthritis. “There’s reason to believe that’s possible,” he speculates.